The invention relates generally to clothes dryers such as, for example, commercial dryers used in self-service laundries. In a conventional dryer, heated air is forced through a rotating drum for evaporating moisture from damp clothes tumbling therein. Air expelled from the drum is then channeled to an exhaust outlet through a duct or series of ducts.
In an effort to conserve energy, many dryers incorporate an additional duct to recirculate a portion of the expelled air back into the drum. However, drying time imposes a limit on the amount of moist air which may be recirculated. Both commercial and residential dryers have therefore limited the amount of recirculated air to less than 60% of the total air entering the drum.
To further conserve energy, various systems have been employed to transfer heat from a dryer exhaust to heat water for use in washing machines. For example, U.S. Pat. No. 4,275,510 discloses a heat pipe coupled between the exhaust of a dryer and the feed water of a water heater for preheating water. U.S. Pat. No. 4,412,391 discloses a heating chamber positioned in the underside for a conventional hot water tank and coupled to the exhaust of a dryer for pre-heating wash water. However, the previous systems have only been able to pre-heat wash water. They have been unable to heat the water to normal water supply temperatures due to the relatively low .DELTA.T between dryer exhaust air and water supply.
Another problem exists for heat recovery from dryers wherein combustion gases from gas burners enter the dryer drum. Combustion gases such as, for example, hydrogen chloride, carbon dioxide, nitrogen dioxide and hydrogen fluoride may combine with water vapor condensate to respectively form hydrochloric acid, carbonic acid, nitric acid and hydrofluoric acid. These acids may corrosively attack heat exchangers which are coupled to the dryer exhaust.
The conventional approaches to clothes drying and heat recovery have an additional disadvantage. That is, the ducts required to exhaust air and recirculate air have added to the size, complexity and thermal mass of the dryer. Increased thermal mass prolongs warm up time, reduces drying efficiency and dissipates exhaust heat which may otherwise be recovered.